(1) Field of the Invention
The present invention generally relates to a white reference data generating unit applied to a shading correction system, and more particularly to a white reference data generating unit applied to a shading correction system which is provided in an optical reader used in digital copy machines, facsimile machines and the like.
(2) Description of Related Art
In an optical reading apparatus used in facsimile machines, digital copy machines and the like, a line sensor such as a CCD (Charge-Coupled Device) sensor optically scans a document so that an image formed on the document is read. This type of line sensor generally has thousands of photosensitive elements. Thus, an image for one line is divided into thousands of pixels, and then the line sensor outputs an image signal having levels corresponding to densities of respective pixels.
In the optical reader, there are an intensity variation of a light irradiating the document, a variation of luminous intensity distribution in a optical system, a sensitivity variation of respective photosensitive elements of the line sensor and the like. Due to the above various variations, there is a case where levels of the image signal corresponding to pixels having the same density differ from each other. To eliminate this problem, a shading correction process is carried out in the optical reading apparatus.
In the shading correction process, white reference data corresponding to each of the photosensitive elements (the pixels) is generated based on the image signal output from the line sensor when the line sensor scans a white reference plate having a uniform white density before scanning the document. When the document is scanned by the line sensor, image data for each pixel obtained based on the image signal output from the line sensor is corrected by use of corresponding white reference data which has been obtained as described above. That is, a ratio of a level of the image signal corresponding to each pixel to the corresponding white reference data is used as the image data for each pixel.
The white reference data for each pixel is obtained by scanning the white reference plate. Thus, if the line sensor scans the white reference plate in a condition in which dust is adhered to the white reference plate, the white reference data corresponding to each pixel on an area to which the dust is adhered is damaged. If the shading correction is carried out based on the white reference data which is damaged by the dust, a white line is formed at a position of each pixel corresponding to the damaged white reference data on an image reproduced based on the image data.
Conventionally, to prevent the white line from forming on the reproduced image, a white reference data generating unit disclosed in Japanese Patent Laid Open Publication No-62-189871 has been proposed. In this conventional white reference data generating unit, a maximum level of levels of the image signals, corresponding to each pixel, obtained by scanning the white reference plate for a plurality of lines is defined as the white reference data for each pixel. According to the conventional white reference generating unit, as the maximum level of the image signal obtained by scanning the white reference plate is used as the white reference data, the white line can be prevented from forming on the reproduced image.
However, as various noises are included in the image signal output from the line sensor, when the level of the image signal instantly increases caused by a noise at a time of scanning the white reference plate, an accurate white reference data cannot be obtained. In this case, a good shading correction cannot be carried out.